1. Field of the Invention
The present invention relates to storage tanks with floating roofs, and more particularly to an improved apparatus for sealing the rim space between an inner wall of a storage tank and the periphery of an internal floating roof within the storage tank.
2. Description of the Prior Art
Bulk fluids such as petroleum and fuel products are usually stored in large cylindrical tanks. These are commonly equipped with internal floating roofs to minimize product losses to the atmosphere. A critical part of the internal floating roof is the sealing mechanism that is installed in the interstitial space between the internal floating roof and the inner wall of the storage tank. Because the internal floating roof has a circumferential edge or "rim", this annular space is commonly called the "rim space." This sealing mechanism is designed to allow the floating roof to stay afloat on the stored product, to maintain a vapor seal, to move easily within the tank as the product levels rise and fall, and to keep the internal floating roof centered within the tank.
The sealing ring, or mechanical shoe type seal, has proven over the years to be one of the more effective types of sealing device. The mechanical shoe type seal consists of a sealing ring that completely encircles the inner periphery of the storage tank wall. This sealing ring is formed of multiple segments or "shoes" that overlap or are connected by some form of expansion joint. This configuration allows for expansion and contraction of the sealing ring segment joints as the tank diameter (and therefore the circumference) varies due to fabrication tolerances and foundation settlement.
The sealing ring is generally supported by hanger-bars or linkages, and various designs have been created and implemented for this purpose. The hanger-bars or linkages are mounted at one end to the side of the internal floating roof's rim which faces outward (toward the wall of the tank), extend on an incline toward the tank wall, and are connected at a second end to the shoe segments. Because the sealing ring supports are mounted at some angle, the vertical forces (frictional and gravitational) acting on the shoe segments transfer the force in part as horizontal loadings, into the lower edges of the rim plate.
The most common form of support is the inclined hanger-bar attached near the upper third of the metal shoe and at or below the lower edge of the internal floating roofs rim plate. Experience has shown that the maximum practical angle between the inclined hanger-bar and the vertical tank shell is on the order of 30.degree.. If the angle is allowed to become greater than this (get closer to the horizontal) the horizontal force vector against the shoe increases the vertical frictional force and the shoe "jams" against the shell. Once this point is reached it becomes "self-energizing" in that additional vertical force adds to the horizontal force vector and the vertical frictional force increases even more.
To keep inclined hanger-bar supports less than this 30.degree. angle, a lengthy inclined hanger-bar is generally employed. The lower pivot point must be on the order of 20 inches or more below the upper pivot point for typical rim spaces or run the risk of the system jamming. One problem associated with this structure is that the lower pivots, and a substantial portion of the hanger-bars, are located below the liquid level and are not easily accessible for maintenance or repair. In addition, considerably more material is required to construct the hanger-bars in this manner.
Internal floating roofs, particularly lightweight internal floating roofs which are generally made of aluminum, are, by design, fairly flexible devices. Flexure of the lower edge of an internal floating roof moves the lower pivot of the hanger-bars horizontally, effectively increasing the angle between the inclined hanger-bar and the vertical tank shell. Additional braces and stiffeners must be employed to resist this flexure.
Internal floating roofs are also designed to be as shallow as possible, to maximize the capacity of the storage tank. Maximum and minimum liquid levels are dictated by interference between the internal floating roof and a fixed roof structure or the tank's interior bottom piping. In addition to increasing material and construction costs, these braces and stiffeners needed to resist flexure in the lower edge of the floating roof structure may also increase the minimum liquid level by reducing the level to which the internal floating roof may drop. As a result, the effective useable storage capacity of the storage tank is reduced.
Another problem associated with this structure is the elevation of individual elements of the sealing ring also changes, because the travel of the upper end of these inclined hanger-bars describes an arc as the rim space changes. The rim space may increase or decrease due to settlement of the tank foundation, original tolerances, or other factors. With the inclined hanger-bar apparatus, variances in the rim space create alignment and fit-up problems that require additional details and equipment to overcome.
Other forms of support have been used over the years including various forms of "pantograph" or "scissors" linkages with horizontal pivot pins. These all require support from the lower edge of the internal floating roofs rim plate and are all subject to the same "jamming" limitations and other problems associated with the inclined hanger-bar system. If these linkage systems are too shallow, or if the support from the bottom edge of the rim plate moves inward due to flexure of any form, the same "self-energizing" jamming mechanism can occur.
As discussed above, the shoe segments overlap or are connected by expansion joints. The most common methods of constructing expansion joints are: (1) the use of flexible fabric at the end of every shoe segment, where the flexible fabric is riveted or bolted to the shoe's vertical edge, (2) the use of a metal expansion joint formed into each shoe segment, and (3) the use of an overlap at the ends of the shoe segments. In the situation where an overlap is used, various means may be employed to keep the overlap a snug fit.
The fabric expansion joints often present a compatibility and service life problem. The metal expansion joints incorporate U-shapes formed into the shoes that allow gaps or openings that exceed accepted gap criteria and promote product loss and environmental pollution. The overlapped ends is the simplest means of creating a seal between adjacent shoe segments, but the various means used to insure a snug fit often complicate this joint, and may not ensure that product loss is minimized.